Batman/Superman: World's Finest or Superman/Batman: World's Finest feel like better titles to me. But, if it is called Batman vs Superman it is what it is. Butts are going to be in the seats in BIG numbers that's for sure no matter what it's called.

Also if its called first Superman/Batman:World's Finest
then for future sequels can called only World's Finest 2
But I'm more from
Man of Steel 2: TDK over Metropolis (Yeah I know its long but sounds awesome)
Man of Steel 2: World's Finest
Man of Steel 2: Public Enemies